Philip Withnall e4a690f5dd gcancellable: Fix minor race between GCancellable and GCancellableSource
There’s a minor race condition between cancellation of a `GCancellable`,
and disposal/finalisation of a `GCancellableSource` in another thread.

Thread A                               Thread B
 g_cancellable_cancel(C)
 →cancellable_source_cancelled(C, S)
                                       g_source_unref(S)
                                       cancellable_source_dispose(S)
 →→g_source_ref(S)
 →→# S is invalid at this point; crash

Thankfully, the `GCancellable` sets `cancelled_running` while it’s
emitting the `cancelled` signal, so if `cancellable_source_dispose()` is
called while that’s high, we know that the thread which is doing the
cancellation has already started (or is committed to starting) calling
`cancellable_source_cancelled()`.

Fix the race by resurrecting the `GCancellableSource` in
`cancellable_source_dispose()`, and signalling this using
`GCancellableSource.resurrected_during_cancellation`. Check for that
flag in `cancellable_source_cancelled()` and ignore cancellation if it’s
set.

The modifications to `resurrected_during_cancellation` and the
cancellable source’s refcount have to be done with `cancellable_mutex`
held so that they are seen atomically by each thread. This should not
affect performance too much, as it only happens during cancellation or
disposal of a `GCancellableSource`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1841
2020-02-29 20:15:39 +00:00
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